Manufacturing a set of mating parts, e.g., a sleeve with a bore and a spool valve that is slidably received in the bore, with very small clearances is challenging, particularly with respect to low-cost parts. Typically, in order to achieve very small clearances, a batch of one part, e.g., Part A, having a specified size and certain geometric tolerances, is made and then a batch of its corresponding part, e.g., Part B, having a specified size and certain geometric tolerances, is made. Under statistical theory, both parts A and B, individually, would exhibit a normal distribution around a mean, with an equal number of parts in the batch being higher and lower than the average part size. Thus, the parts could be “matched,” i.e., a Part A could be matched with a Part B so that a very small clearance is achieved when assembled. While in theory this should result in little wasted material and involve only the physical process of identifying parts that should be joined together, for parts requiring a very low clearance this process is costly, inefficient, and is not always successful in achieving the desired clearance.
Manufacturers have sought more efficient and effective processes to achieve low clearance matching parts with limited success. Processors have attempted to specify the limits of geometric error on the two parts, thus establishing a maximum clearance amount. However, this methodology does not guarantee that the two parts will fit together, for example, if a Part A is at the upper limit of its geometric tolerance Part A may not fit together with a Part B at the lower end of its geometric tolerance. In addition, the parts may not fit together with a sufficiently low clearance. For example, if both Part A and Part B are at the low end of their respective geometric tolerance ranges, the clearance between the two parts may be higher than desired. Further, to ensure the desired clearance is achieved and to reduce assembly costs, Parts A and B must generally be manufactured to a high degree of precision to reduce the amount of variation, which increases manufacturing costs considerably. Given these constraints, previous manufacturing techniques that were capable of rapid throughput, i.e., less than 10 minute production times, were generally only able to achieve clearance tolerances of approximately 0.0011 inch.
The complexity and difficulty in manufacturing low clearance mated parts is exacerbated when attempting to create a set of mated parts when, for instance, Part A has a shape that includes two or more diameters, e.g., a first diameter extending for a certain portion of Part A and a second diameter extending for another portion of Part A. Multiple diameter parts pose challenges to manufacturers because Part A generally needs to mate with Part B at a low clearance at both diameters. However, the first diameter portion of Part A may end up at a certain tolerance and the second diameter portion of Part A may have a different tolerance. Thus, typically attempts to mate Part A with Part B require a compromise between the clearance at the first diameter portion and the clearance at the second diameter portion, resulting in a set of parts without an overall desired clearance value.
In addition, the aforementioned processes and others known in the art do not account for the challenges that arise when parts are treated to increase hardness. For example, hardcoat anodizing is a process that is generally used to increase the wear and corrosion resistance of the natural oxide layer on the surface of aluminum parts. A typical hardcoat anodizing application will add several thousandths of an inch to the surface of a part. However, the process typically does not provide a repeatable thickness from part to part and thus the tolerance range on a batch of parts achieved during the original machining process may be lost. While some manufacturers will machine the hardcoated part in order to return a part to within required tolerances, machining the hardcoated part is not only an extra, costly process, it can damage the hardcoat, thus limiting its effectiveness.